Heat-induced changes in intracellular Na+, pH and bioenergetic status in superfused RIF-1 tumour cells determined by 23Na and 31P magnetic resonance spectroscopy

The acute effects of hyperthermia on intracellular Na+ (Na-i(+)), bioenergetic status and intracellular pH (pH(i)) were investigated in superfused Radiation Induced Fibrosarcoma-1 (RIF-1) tumour cells using shift-reagent-aided Na-23 and P-31 nuclear magnetic resonance (NMR) spectroscopy. Hyperthermia at 45 degrees C for 30 min produced a 50% increase in Na-i(+), a 0.42 unit decrease in pHi and a 40 - 45% decrease in NTP/P-i. During post-hyperthermia superfusion at 37 degrees C, pH(i) and NTP/P-i recovered to the baseline value, but Na-i(+) initially decreased and then increased to the hyperthermic level 60 min after heating. Hyperthermia at 42 degrees C caused only a 15 - 20% increase in Na-i(+). In the presence of 3 mu M 5-(N-ethyl-N-isopropyl) amiloride ( EIPA), an inhibitor of the Na+/H+ exchanger, the increase in Na-i(+) during 45 degrees C hyperthermia was attenuated, suggesting that the heat-induced increase in Na-i(+) was mainly due to an increase in Na+/H+ anti-porter activity. EIPA did not prevent hyperthermia-induced acidification. This suggests that pHi is controlled by other ion exchange mechanisms in addition to the Na+/H+ exchanger. EIPA increased the thermo-sensitivity of the RIF-1 tumour cells only slightly as measured by cell viability and clonogenic assays. The hyperthermia-induced irreversible increase in Na-i(+) suggests that changes in transmembrane ion gradients play an important role in cell damage induced by hyperthermia.